A number of systems have been developed over the years to provide multi-temperature refrigeration for the transport of perishable or temperature-sensitive goods in large trailers or containers. The systems are generally designed to work both while the trailer is parked and during operation on the road. These systems, referred to as “trailer refrigeration units” (TRU) in the trucking industry, are predominantly vapor compression cycle (VCC) refrigeration units driven by diesel auxiliary power unit (APU) engines (directly or through a gen-set) for large 53-foot trailers. On smaller trucks the refrigeration systems may be powered by truck alternator-produced electrical energy or by APU diesel engines. The temperature ranges required for transport of the most common refrigerated goods generally range from about −20° F. to about 40° F. The cooling capacity must be provided for at least several hours when operating over the road.
Existing VCC TRU systems with diesel APUs have been used for many years. However, APU-powered TRU systems have some significant shortcomings. First, these systems are characterized by high operating costs due to their large consumption of diesel fuel. Second, operation by use of diesel fuel raises a number of regulatory issues, including noise restrictions and proven health critical emissions from the diesel engines.
Electrical trailer refrigeration units powered by truck alternators are another one of the systems providing multi-temperature refrigeration, but these systems have been generally limited to smaller trucks and box trucks. In other applications, such as with a large 53-foot trailer, these systems have proven nonviable because of the constraints of delivering power from the truck to the trailer due to impractical power transfer and operational logistics issues. The impracticalities in power transfer from the truck include the significant power generation cost and issues in packaging the systems on the truck itself. There are also significant concerns with transferring high voltage or current from the truck to the trailer. In the shipping industry there is often a split in the ownership of the vehicle and the container it is transporting. For example, with semi-trucks, one party often owns the truck, or tractor, while another party owns the trailer. This split ownership creates operational logistics problems for the delivery of power from the truck to the trailer. And trailers outnumber trucks by a factor of two or three to one. Therefore, implementing truck to trailer power becomes impractical, as the truck must be properly equipped to power the refrigeration system employed by the trailer. And there would need to be sufficient numbers of trucks to power all trailers needing power.
Eutectic solid plate, or cold plate, refrigeration systems are another type of system employed by smaller delivery trucks. Cold plate systems rely on heat transfer to large, solid plates, with the heat absorbed by the cold plate cooling the surrounding air. Cold plate systems are charged by shore power-driven (i.e. electric plug) vapor compression cycle refrigeration systems. These systems are often used for trucks designed for local delivery and are characterized by much less precise temperature control. These systems have many short-comings, including their limited heat absorbance capacity, long recharge times for the eutectic plates, the limited temperature control features of the systems and the excessively heavy weights of the plates. This has generally limited the application of cold plates systems to smaller, local delivery trucks.
Battery-powered trailer refrigeration units represent a third type of system, though with more limited application than the truck alternator-powered TRU or cold plates. Battery-powered trailer refrigeration units could be charged by shore power and many other means, including power provided by the truck APU. While these systems enable diesel-free operation over the road, their initial cost is prohibitively expensive, they are impractically heavy to use for large refrigerated trailer systems, and battery replacement cost further limit their economic feasibility.
Large refrigerated containers, such as sea containers, generally utilize VCC refrigeration units that are powered by high voltage electricity supplied by the vessel when aboard ships and can be powered from shore power during transit at ports. Sea containers using electrically-powered VCC place a particularly large current demand on the system's electrical grid aboard ship or from shore power when the unit cycles. The sea container systems also require significant steady state power requiring oversized power networks or very complex systems to manage the number of units operating and the timing of the cycling.
The considerations described above highlight the economic importance of developing more efficient cooling systems. In addition to these economic factors, regulatory restrictions will create additional incentives to adopt more efficient systems. Recent regulatory activity in the U.S. relating to the diesel engine on TRU units has resulted in additional administrative and economic burdens for fleets operating diesel-powered TRU units. Specifically, the environmental protection agency (EPA) and the Air Resources Board of California (CARB) have enacted legislation requiring reporting and upgrading the diesel engines and or emission after treatment equipment used on trailer refrigeration units. California law, as adopted in the CARB/EPA tier IV requirements of 2013, requires the business entities that arrange, hire, contract for, or dispatch TRU-equipped trucks, trailers, shipping containers, or railcars for transport of perishable goods on California highways and railways to require the motor carriers to dispatch only TRUs that comply with the TRU Regulation. This legislation, in addition to the direct operational fuel costs and indirect emission and noise impacts, provide significant incentives for the elimination of the diesel engine from the TRU.
Given the number of short-comings of the systems currently in operation, including the reliance of many systems on diesel power, the coupling of the trailer to the truck for power generation, and the high weight of some systems, there exists a strong and well-defined need for more efficient cooling systems for the transport of perishable or temperature sensitive goods in large trailers or containers. The present invention provides systems to meet these important needs as detailed in the following disclosure.